The entire disclosure of Japanese Patent Application No. P2002-185129 filed on Jun. 25, 2002 and Japanese Patent Application No. P2003-120518 filed on Apr. 24, 2003 including specification, claims, drawings, and summary is incorporated herein by reference in its entirety.
1. Field of the Invention
This invention relates to a fault diagnosis apparatus of a fuel evaporation/dissipation prevention system.
2. Description of the Related Art
A fuel evaporation/dissipation prevention system is provided in an automobile to prevent emission of evaporated fuel occurring inside a fuel tank into the atmosphere. The fuel evaporation/dissipation prevention system includes a canister, a vapor passage extending between a fuel tank and the canister and having a purge valve fitted thereto, and a purge passage extending between the canister and an intake passage of an internal combustion engine. The canister adsorbs the evaporated fuel inside the fuel tank through the vapor passage. On the other hand, the purge valve is opened under a predetermined condition so that the evaporated/dissipated fuel adsorbed to the canister can be purged into the intake passage of the internal combustion engine through the purge passage.
A fault diagnosis apparatus for detecting leak abnormality of the fuel evaporation/dissipation prevention system is provided to this prevention system. The fault diagnosis apparatus includes a vent valve fitted to the canister, a pressure sensor for detecting an internal pressure of the fuel tank, and an electronic control unit (ECU) for inputting detection information from the pressure sensor and controlling opening/closing of the vent valve and the purge valve. To make fault diagnosis, the fault diagnosis apparatus opens the purge valve and closes the vent valve to bring the fuel tank, the vapor passage, and the purge passage as the fault diagnosis object regions of the fuel evaporation/dissipation prevention system into a predetermined negative pressure state. The fault diagnosis apparatus then closes the purge valve, measures the internal pressure of the fuel tank while the fault diagnosis object regions are thus closed, and judges that leak abnormality exists when an increment of the tank internal pressure is greater than a judgment value.
However, because the increase of the tank internal pressure occurs owing to various factors, an erroneous judgment is likely to be made when leak judgment is made on the basis of the comparison result of the increment of the tank internal pressure with the judgment value. One of the causes of the increase of the tank internal pressure is that external air flows into the tank through a small hole formed in the fuel tank. On the other hand, even when no leak occurs in the fuel tank, the tank internal pressure rises due to evaporation/dissipation of the fuel when the degree of evaporation saturation of the fuel inside the tank is low. The fuel is returned from the internal combustion engine into the fuel tank through a low-pressure fuel passage. This return fuel is also a cause of the increase of the evaporation dissipation amount. Particularly, when the fuel remaining amount becomes small inside the tank, evaporation/dissipation of the fuel due to this return fuel becomes remarkable inside the tank. A winter fuel is used at cold places from fall to spring. Because the winter fuel has a larger content of alcohol, that is, a larger evaporation/dissipation amount, fuel evaporation/dissipation is remarkable particularly on warm days. The causes of the increase of the internal pressure of the fuel tanks as criteria of the fault diagnosis can thus be divided into the leak hole and fuel evaporation/dissipation. In order to make a correct fault diagnosis, it is therefore necessary to correctly judge the causes of the increase of the tank internal pressure.
Therefore, fault judgment is tentatively made when the increment of the tank internal pressure, measured under the closed condition after the fault diagnosis object region, is brought into a reduced pressure state exceeds a first judgment value, then the increment of the tank interval pressure is measured while the fault diagnosis object region is released to the atmosphere and then closed, and final judgment is then made by comparing this measurement value with a second judgment value. In other words, when the increment of the tank internal pressure after the release to the atmosphere is smaller than the second judgment value, final judgment is made to the effect that a leak hole exists. When the increment of the tank internal pressure is greater than the second judgment value, on the other hand, the tank internal pressure is judged as increasing due to evaporation/dissipation of the fuel. In this case, the tentative fault judgment is withdrawn and final judgment is made to the effect that existence/absence of the leak hole is not known (diagnosis result by high evaporation/dissipation judgment is invalidated).
Demands have increased in recent years to prevent ultra-trace amount leak in the fuel evaporation/dissipation prevention system. Ultra-small leak holes as the main cause of this ultra-trace amount leak have diameters of about 0.5 mm. On the other hand, the small leak holes that have so far been the object of detection have diameters of about 1.0 mm, and the diameters of both holes are remarkably different. When the leak holes having different diameters are the objects of detection in fault diagnosis of the fuel evaporation/dissipation prevention system, it becomes more difficult to correctly discriminate whether the increment of the tank internal pressure results from the leak holes or from evaporation/dissipation of the fuel. In other words, the smaller the diameter of the hole, the smaller becomes the increment of the tank internal pressure resulting from the leak hole. To discriminate the increase of the tank internal pressure resulting from the ultra-small leak hole from the increase of the tank internal pressure resulting from evaporation/dissipation of the fuel, the second judgment value must be lowered. When the second judgment value is set to a smaller value, however, the increment of the tank internal pressure under the closed state after the release to the atmosphere is likely to exceed the second judgment value. Therefore, even when the small leak hole exists and is tentatively judged as existing, the number of cases where the increment of the tank internal pressure under the closed state after the release to the atmosphere exceeds the second judgment value and leak judgment is withdrawn becomes greater, and trace amount leak resulting from the small leak hole cannot be detected.
It is an object of the present invention to provide a fault diagnosis apparatus capable of accurately judging abnormality resulting from trace amount leak and ultra-trace amount leak in the evaporated fuel dissipation prevention system.
In the fault diagnosis apparatus according to the invention, when a first restoring pressure amount measured after a fault diagnosis object region of the fuel evaporation/dissipation prevention system is brought into a reduced pressure state exceeds a first judgment value or a second judgment value greater than the first judgment value, a second restoring pressure amount is measured by sealing the fault diagnosis object region after an atmospheric pressure is introduced into the fault diagnosis object region, the second restoring pressure amount is compared next with a third judgment value when the first restoring pressure amount is greater than the first judgment value but is smaller than the second judgment value, and the second restoring pressure amount is compared with a fourth judgment value greater than the third judgment value when the first restoring pressure amount is greater than the second judgment value. When the first restoring pressure amount is greater than the first judgment value and the second restoring pressure amount is smaller than the third judgment value, or when the first restoring pressure amount is greater than the second judgment value and the second restoring pressure amount is smaller than the fourth judgment value, the fuel evaporation/dissipation prevention system is judged as being abnormal.
The increment of the first restoring pressure amount resulting from the leak hole changes with a leak hole diameter. Therefore, it is difficult to judge existence/absence of the leak holes having various diameters without being affected by evaporation/dissipation of the fuel. The fault diagnosis apparatus according to claim 1 can set, respectively, the first and second judgment values in association with ultra-trace amount leak and trace amount leak (for example, ultra-small leak hole and small hole respectively inducing ultra-trace amount leak and trace amount leak) and can also set the third and fourth judgment values so that abnormality resulting from ultra-trace amount leak and trace amount leak can be distinguished from abnormality resulting from evaporation/dissipation of the fuel. Therefore, ultra-trace amount leak and trace amount leak can be distinguished from the increase of the restoring pressure amount resulting from the fuel evaporation/dissipation and can be correctly judged on the basis of the restoring pressure amount.
In other words, when the first restoring pressure amount exceeds the first judgment value as the judgment criterion of ultra-trace amount leak but is smaller than the second judgment value as the judgment criterion of trace amount leak, abnormality resulting from ultra-trace amount leak is judged tentatively. Next, the second restoring pressure amount is measured in order to discriminate whether such an increase of the first restoring pressure amount results from ultra-trace amount leak or from evaporation/dissipation of the fuel. When the second restoring pressure amount exceeds the third judgment value, evaporation/dissipation of the fuel is judged as being the cause of the increase of the first restoring pressure amount. In consequence, tentative judgment of ultra-trace amount leak abnormality is withdrawn, and final judgment is made to the effect that existence/absence of ultra-trace amount leak is not known (diagnosis result by high evaporation/dissipation judgment is invalidated). When the second restoring pressure amount does not exceed the third judgment value, on the other hand, ultra-trace amount leak is judged as being the cause of the increase of the first restoring pressure amount, and ultra-trace amount leak abnormality is finally judged.
When the first restoring pressure amount exceeds the second judgment value, abnormality resulting from trace amount leak is judged tentatively. Next, the second restoring pressure amount is measured to discriminate the cause of the increase of the first restoring pressure amount. When the second restoring pressure amount exceeds the fourth judgment value, evaporation/dissipation of the fuel is judged as being the cause by the increase of the first restoring pressure amount, and final judgment is made to the effect that existence/absence of trace amount leak is not known (diagnosis result by high evaporation/dissipation judgment is invalidated). When the second restoring pressure amount does not exceed the fourth judgment value, on the other hand, trace amount leak is judged as being the cause of the increase of the first restoring pressure amount, and trace amount leak abnormality is finally judged as existing.
As described above, the fault diagnosis apparatus according to the present invention can correctly judge ultra-trace amount leak and trace amount leak, respectively, resulting from the ultra-small leak hole and the small leak hole.